The present invention relates to a method for upgrading a gas by separation of carbon dioxide therefrom.
Most gases employed for large-scale energy purposes, such as natural gas, town gas, and biogas, present as their main constituent methane. Various other combustibles may also be present, and additionally a certain share of carbon dioxide is normally encountered, which latter compound may com-promise the technical and calorific properties of the gas.
On the other hand, the bulk of flue gases arising from power plants and a range of other sources is typically made up of free nitrogen. Here, carbon dioxide is captured out of climate concerns or to recover it as a valuable product in its own right.
When it comes to the initially mentioned gases for energy purposes, they must be upgraded to a methane content of 95-98% in order to meet the standards required for gaining entrance to the natural gas grid and to be accepted as vehicle fuels. In this regard, carbon dioxide, which, most pro-nounced in the case of biogas, may constitute as much as 45% of the crude gas and acts to dilute its energy content, must be cleared away to a large extent.
To separate carbon dioxide from methane, various methods have been applied, among which water scrubbing and pressure/temperature swing adsorption are prominent.
Water scrubbing relies on the fact that carbon dioxide is more soluble in water than methane. The absorption process is purely physical. Normally, the gas is pressurised and fed to the bottom of a packed column, while a stream of water is introduced to the top of the column so that the absorption process is operated counter-currently. The spent water typically must be stripped with air in another column in order to desorb the absorbed carbon dioxide.
In the case of flue gas, wherein carbon dioxide normally is to be separated from a bulk of N2 rather than CH4, a favourite absorbent has been mo-noethanol amine (MEA) in stead of water.
Pressure/temperature swing adsorption makes use of adsorbent materi-als, for which carbon dioxide shows a selective affinity. Under pressure or low temperature, carbon dioxide tends to be attracted to certain solid surfaces more strongly than methane. When the pressure is subsequently reduced or the temperature is raised, the carbon dioxide is desorbed and can be re-moved.
The absorption of carbon dioxide in water as in water scrubbing and its adsorption to a solid material during pressure/temperature swing adsorption are generally perceived in the art as two distinct and antagonistic methods to be practiced separately.
For example, the British patent GB 1296889, which discloses a method for separation of carbon dioxide from other gases by temperature swing adsorption to an ion exchange resin, teaches that the resin when adsorbing carbon dioxide should not be wet with unsorbed water as this is believed to hin-der carbon dioxide adsorption. In this regard a content of water in the bed of resin of less than 30% is specified as being desirable.
Likewise, the international patent application WO 2011/049759 de-scribes a method for removing carbon dioxide from a gas stream by temperature and optionally pressure swing adsorption to an ion exchange resin. It is stated that a water content of above 10% by weight in the resin is not bene-ficial and unnecessarily increases regeneration heat requirements.
In the patent application US 2005/0160913 concerning a carbon dioxide absorbent mainly intended for a rebreather system, resort is made to a strongly basic ion exchanger in the form of lithium hydroxide. Said compound is pre-hydrated to form its monohydrate, i.a. to forestall an exothermic reac-tion when in use. When hydrated to a stoichiometric extent, the content of water amounts to approximately 43% by weight. Accordingly, at this level all water is intimately incorporated into LiOH as water of hydration. Thus, no unsorbed water is present, and it is stated that pre-moistening of the ion exchanger beyond this stage is generally undesirable since the solid LiOH would otherwise begin to dissolve and lose its properties.
As adumbrated in the above mention of water scrubbing, a disad-vantage connected to that method is the need for subsequent treatment of the spent water in a separate procedure.
On the other hand, the alternative method of pressure/temperature swing adsorption is not free from incommodities, either. During the phase of its regeneration by pressure and or temperature swing, the adsorbent material obviously is not available for adsorption of carbon dioxide. Accordingly, high demands are set for the adsorption capacity of the adsorbent material and often it must be desorbed too frequently for the method to be operation-ally and economically viable.